1. Technical Field
This invention relates to sheet inverters for use with a sheet handling equipment, and more particularly, to a highly efficient and reliable high-speed sheet inverter mechanism and method for inverting sheets particularly suited for use in a high-speed copier or printer.
2. Background Art
As disclosed, for example, in U.S. Pat. Nos. 4,673,176 and 4,842,262, sheet inverter mechanisms are well known for use in electrostatographic copiers and printers. Such inverter mechanisms are used to reverse the lead and trail edge, and hence the front-and-back face, orientations of a sheet being used for imaging thereon in the copier or printer.
Typically, sheet inverter mechanisms include a three-roll assembly and a curved narrow sheet turnover or confinement chamber with sheet recoil means therein. The roll assembly forms sheet entrance and exit nips to the chamber. A sheet is fed into the chamber through the entrance nip, lead edge first, and its trail edge is then "walked" by the roller assembly from the entrance nip to the exit nip. The roller assembly thereafter then feeds the sheet through the exit nip out of the chamber, trail edge first, thereby reversing the lead and trail edge orientations of the sheet.
Many things can and do go wrong with a sheet being inverted as such. As a consequence, the art of sheet inverters has become highly developed. Things go wrong because different size sheets, for example, may be used, or because the dimensions of a particular size sheet may vary from sheet-to-sheet, and so also may the exact positioning of the roller assembly and the recoil means. The undesirable results, for example, may include trail edge jams and misfeeds. Finger and paddle assemblies, as disclosed for example in U.S. Pat. Nos. 4,842,262 and 4,842,263 have been proposed for overcoming such resulting trail edge jams and misfeeds.
Additionally, the weight and beam strength of sheets may vary, thus, for example, introducing variable and premature buckling of sheets within the turnover or confinement chamber. For overcoming such problems, corrugation roller assemblies and buckle-spring assemblies, for examples, have been proposed in U.S. Pat. Nos. 4,673,176 and 4,262,895. Finally, other prevalent problems occur, for example, sheet skewing, lack of precise sheet control, and sheet interference within the turnover or confinement chamber, especially in turnover mechanisms used with high-speed equipment. To reduce these problems, special chamber geometry, and sheet-entrance and sheet-exit sensors for examples have been proposed in U.S. Pat. Nos. 4,531,725 and 4,214,740.
The above problems, for examples, are particularly serious in high-speed equipment such as copiers or printers which run at 100 sheets or more per minute. In order for the inverter to keep up with such a copier or printer, it has been suggested, for example, in the aforesaid 4,673,176 patent, to run consecutive sheets simultaneously through the turnover or confinement chamber thereby increasing the sheet throughput rate of the inverter. Alternatively, the inverter can be speeded up so as to continue to invert sheets singly through the chamber. Speeding up the inverter, however, has been found to introduce additional and unacceptable reliability problems.
On the other hand, simultaneous sheet handling at high speeds has been found to involve serious interference problems between the entering sheet and the exiting sheet. For example, the trailing edge of the exiting sheet may be caught and trapped in the chamber by an entering sheet, or the lead edge of an entering sheet may be caught and directly diverted into the exit nip by the exiting sheet. At such high speeds, the trail edge of the entering sheet may fail for various reasons to reliably and efficiently go from the entrance nip to the exit nip. For example, variations in sheet size and weight may make the "walking over" approach to trail edge transfer from the entrance nip to the exit nip, inefficient and unreliable.